1. Field of the Invention
The present invention relates to a process for the preparation of an aqueous polymer dispersion by free radical aqueous emulsion polymerization of at least one ethylenically unsaturated compound (monomer) in the presence of at least one dispersant.
The present invention also relates to the aqueous polymer dispersions obtainable by the novel process, the use thereof as binders in adhesives, sealing compounds, plastics renders, paper coating slips and surface coating materials and for modifying mineral binders.
2. Description of the Related Art
Aqueous polymer dispersions (latices) are generally known. They are fluid systems which comprise polymer coils consisting of a plurality of entangled polymer chains (i.e. polymer particles) as the disperse phase in an aqueous dispersing medium. The weight average diameter of the polymer particles is frequently from 10 to 5,000 nm.
As in the case of polymer solutions on evaporation of the solvent, aqueous polymer dispersions, on evaporation of the aqueous dispersing medium, have the potential for forming polymer films and are therefore used in particular as binders. Owing to their environmentally friendly properties, they are becoming increasingly important.
The preparation of aqueous polymer dispersions is frequently carried out by free radical aqueous emulsion polymerization [cf. for example Encyclopedia of Polymer Science and Engineering, Vol. 8, page 659 et seq. (1987); D. C. Blackley, in High Polymer Latices, Vol. 1, page 35 et seq. (1966); H. Warson, The Applications of Synthetic Resin Emulsions, page 246 et seq., Chapter 5 (1972); D. Diederich, Chemie in unserer Zeit, 24, (1990), 135 to 142; Emulsion Polymerisation, Interscience Publishers, New York (1965); DE-A 40 03 422, and Dispersionen synthetischer Hochpolymerer, F. Holscher, Springer-Verlag, Berlin 1969]. A characteristic of this process is that water-soluble free radical initiators are used as polymerization catalysts.
For reasons relating to apparatus, the free radical aqueous emulsion polymerization is frequently carried out at below 100°. Since the polymerization rates [corresponding to the instantaneous monomer conversion rate] decrease sharply with decreasing monomer concentration and, on the other hand, the operation times of the polymerization reactors, which for reaction safety reasons are equipped with expensive instrumentation, are to be minimized, the polymerization reaction in the polymerization vessel is frequently carried out only to a monomer conversion of ≧95% by weight. The conversion of the remaining monomers is then effected in a downstream polymerization step in a separate reactor generally equipped with less expensive instrumentation. As a rule, this postpolymerization is carried out using a free radical initiator or free radical initiator system optimized for these purposes. Examples of free radical initiators which are particularly suitable for the postpolymerization or reduction of residual monomer contents are to be found in EP-B 003 957, EP-B 028 348, EP-B 563 726, EP-A 764 699, U.S. Pat. No. 4,529,753, DE-A 37 18 520, DE-A 38 34 734, DE-A 42 32 194, DE-A 195 29 599, WO 95/33775, EP-A 767 180, DE-A 19839199 or DE-A 19840586.
In addition, WO 00/22003 discloses a one-stage process for the preparation of aqueous polymer dispersions having a low residual monomer content, in which process the polymerization reaction is initiated by a redox initiator system and the polymerization temperature is specified by a defined temperature profile. A disadvantage of this process is that, in addition to the oxidizing agent acting as a free radical initiator, it is essential for a reducing agent to be present, by means of which the polymerization temperature can be reduced but through which additional foreign components also enter the aqueous polymer dispersions, which foreign components generally cannot be removed from the aqueous polymer dispersion even by subsequent stripping with inert gas or with steam.
Against the background of the prior art, it is an object of the present invention to provide a further one-stage process for the preparation of aqueous polymer dispersions having a low residual monomer content, which process, however, manages without additional reducing agents.